1. Field of Invention
The present invention relates to copper-alloy and bearing having improved seizure resistance. More particularly, the present invention relates to a copper alloy having improved seizure resistance over that of a conventional KELMET, i.e., a Cu--Pb--Sn alloy, as well as a bearing, in which the improved copper-alloy is used.
2. Description of Related Art
KELMET, which is a representative conventional copper-based sliding material, is usually coated with a 10- to 20 .mu.m-thick overlay consisting of soft metal or resin and is used as engine parts. In the initial period of use of the sliding bearing, the overlay wears out in such a manner that its surface conforms to that of the opposed shaft, with the result that seizure between the shaft and the sliding bearing becomes difficult to occur. One of the functions of the overlay resides, therefore, in ensuring the compatibility between the shaft and the sliding bearing.
Furthermore, it is usually carried out to form a Ni plating as the underlying layer of the overlay. The Ni plating and the KELMET are referred to as the Ni barrier and the lining, respectively. When the overlay is totally worn out to expose the underlying Ni barrier or even the lining, the seizure becomes readily to occur. In the conventional bearings, the overlay has, therefore, been deposited more thickly than the level required for the compatibility.
Lead particles contained in the KELMET are elongated by the opposed shaft and form a soft film on the sliding surface. The lead particles exhibit, therefore, a function to prevent seizure. This function alone is, however, inadequate under the recent sliding condition which becomes more and more severe. Therefore, proposals have been made to enhance the seizure resistance of the conventional KELMET. They are: adding a Cu-matrix enhancing element such as P, Al or the like; adding Bi which has good compatibility; adding a seizure-enhancing component such as graphite; and adding a wear-resistance enhancing component such as Al.sub.2 O.sub.3 ; and, impregnating the sintered material with resin. These proposals have attained some degree of success.
Engine oil, transmission oil, gear oil or the like are the lubricating oil which lubricates between the above-mentioned sliding materials and the opposite material. The sulfur-based additives are added to these oils.
First, the additives to the gasoline-engine oil are dialkylmonosulfide for preventing the oxidative deterioration of the engine oil, a sulfonate- or phenate- base metal detergent agent for cleaning the sludge formed by oxidation of the engine oil, a dithiophosphate molybdenum compound or a dithiocarbamate molybdenum compound for preventing foaming of the low-viscosity engine-oil, or the like. The above-mentioned dialkylmonosulfide is believed to tonically decompose the hydroperoxide which is formed by the oxidation of the base oil. However, detrimental effects of the additives are also pointed out. For example, the metal-based detergent agent forms sulfate-ash or sludge. The amount of its use is, therefore, limited. In addition, it is said that the foaming-preventing agent may exert a detrimental effect on the performance of the bearing metal.
ZnDTP (dialkyldithiozincphosphate) is added to the diesel-engine oil as a countermeasure against the wear due to soot. Into the rotary-engine oil, olefin sulfide, sulfurized oil or the like is added as the sulfur-based extreme pressure agent, and thiozinc-phosphate, dithiocarbamate molybdenum sulfide is added as the organic-metal anti-wear agent.
Into the transmission oil and the gear oil, olefin sulfide, sulfurized oil or the like is added as the sulfur-based extreme pressure agent, and thiozinc-phosphate or dithiocarbamate molybdenum sulfide is added as the organic-metal anti-wear agent, phosphate-esteramino-salt as the phosphorus-base anti-wear agent. The sulfur concentration ranges from 0.37 to 1.7% in the commercially available oil at present.
It is known that, when the above-mentioned various oils detriorate, corrosion problems arise in the copper-based sliding materials. As a corrosion countermeasure, the present applicant filed the following patent applications.
U.S. Pat. No. 4,878,768: The deteriorated oil causes the corrosion of the Pb phase present in the clearances between the skeleton of the Cu--Pb based sintered alloy used as a sliding bearing of a diesel engine. In order to prevent such corrosion, In is added to the Pb phase.
Japanese Unexamined Patent Publication No. 7-118,777: sintered copper-alloy-based composite sliding member, which is based on the sintered copper alloy consisting of more than 15% and up to 40% of Zn, from 0.5 to 6% of graphite, and from 0.5 to 6% of one or more of Al.sub.2 O.sub.3, SiO.sub.2 and Fe.sub.3 P, and the balance being Cu. In this application, Zn is added in the above-mentioned amount to prevent the corrosion which results from the fact that the deteriorated transmission oil forms CuS on the surface of the copper alloy.
When the conventional KELMET is used as the lining, lead contained in the KELMET is corroded by the deteriorated lubricating oil, and the surface of lining is likely to be roughened. Furthermore, lead is dissolved into the lubricating oil, and the lead phases of the KELMET are changed into pores, thereby lessening the strength of the KELMET and thus causing the lining to collapse. The seizure resistance of the conventional KELMET is, therefore, very low.
The present inventors carried out a sliding test of a copper-based sliding material, in which the lubricating oil with the above-mentioned sulfur-based additives was used, and which almost wholly reproduces the usage conditions of an actual machine. It was then recognized that the total acid value of the lubricating oil (diesel-engine oil, CD grade, 10W-30) greately increased as follows.
Time 0(h) 50(h) 100(h) 150(h) 220(h) Total acid number 1.3 6.4 18.0 21.5 22.3 (mg KOH/g) Strong acid number 0 0 0.1 0.2 0.3 (mg KOH/g) Total base number 3.8 0.5 0 0 0 (mg KOH/g)
The corrosion progresses along with the increase in the acid value. In the conventional bearing, the thickness of the overlay deposited on the lining cannot, therefore, be as thin as the value satisfying the compatibility.
The Ni barrier, which has been heretofore used as the underlying layer of the overlay, impedes the diffusion of Sn and In from the overlay into the Pb phases of the lining and has thus a role of maintaining the corrosion resistance of the KELMET at a high level. On the other hand, when the Ni barrier is exposed, the seizure is disadvantageously likely to occur because of poor seizure resistance of Ni.
According to a recently published research ("Tribologist" (in Japanese) Vol. 41, No. 2, 1996, pp 115-120), the surface of metallic material is converted to an amorphous structure due to repeated sliding friction, and the wear resistance of the surface of various metallic materials can be enhanced by utilizing the vitrification phenomena. It is anticipated that the material development from such a point of view will be accelerated. The present inventors have started already extensive studies to enhance the sliding properties of the copper-alloy surface by means of adding particular element(s) and modifying the surface of lining.